The Bible - Numbers 12:3 goes, Now Moses was a very humble man, more humble than anyone else on the face of the earth. And who wrote Numbers? That's right. (Justified, in that the Biblical concept of humility is attributing all success to God rather than your own abilities.)
Double Justified, given how resistant he was to being a prophet and leader multiple times (which gave him vast power), and also the power he initially lost as part of the life he had in the ruling family, to then be a serf, it actually fit his history. If nothing else, 40 years of migration, in a desert, with ungrateful people, all depending on God, and knowing you'll die before even reaching your goal after all that, will humble someone a bit.
Also, there's the simple fact that the word "humble" has multiple meanings. It is often used to mean someone who has a low, or at least not high, opinion of himself. But it can also refer to the material circumstances in which someone lives, as in the phrases "be it ever so humble, there's no place like home" or "humble abode." The Bible may simply be referring to the fact that Moses lived in humble circumstances, because he had not used his position to enrich himself, or otherwise enjoy the perks that usually come with power. And there are still other meanings of "humble" besides that.
The parable of the pharisee and the tax collector in Luke 18 is a straight example of this trope. The pharisee loudly praises himself for not being like that filthy tax collector over there while the tax collector doesn't even dare to lift his eyes to heaven and simply prays, "God, have mercy on me, a sinner."